The door only has 2 slits on top for ventilation and the two built in drains on the bottom. When I constructed the box I have it so that any water will drain out of the bottom holes as seen in the picture below.
The inside of the box is sealed and can handle up to an inch of water if that was to happen. I haven't had the trailer rolling down the highway in the rain yet, but sitting still with the torrential rains we have here in Tampa produced no water in the aluminum box. Not a drop. The frame for the battery door opening is extended to prevent water from running in. Some will obviously blow in but it is the same as was the original. Still, it should be minimal and will still drain right off. It solved my problem of of the busted battery box, and the fact that I wanted more than one battery and my trailer only had the one battery compartment.